1. Field of the Invention
The invention relates to encoders and more particularly to encoders including matrix arrays of electrical sensors.
2. Description of the Prior Art
Encoders are widely used in electrical apparatus to provide a signal indicative of a monitored condition. Generally, an encoder includes a transducer or sensor unit, a detecting unit, and a readout unit. The transducer unit typically includes an array of electrical sensors having variable voltage, current, impedance, frequency, or other electrical parameter which changes in response to a monitored condition. The detecting unit includes both sampling control and detecting circuits to sample the electrical state of the sensors and produce output signals corresponding to the sensor states. These output signals are, in turn, converted by the readout unit into coded output signals representing the status information from the sensors in a desired format. One example of an encoder application is a keyboard for a computer or electric typewriter, which provides an electrical signal indicative of which of the keyboard keys have been activated.
Another application utilizing encoder apparatus is a meter dial encoding register device for use with a watthour meter of the type typically employed to measure the amount of electrical energy consumed by an electric utility customer. The meter has a mechanical register including a plurality of dials which provide a visual indication of the amount of electrical energy consumed. An encoder is provided to translate the position of the dial shafts into an electrical signal such that the same information regarding the consumption of electrical energy as is shown by the dials is transmitted by an electrical signal to electronic apparatus located either at the meter site or at a remote location.
The encoder often consists of an array of photoresistive sensors to detect the presence or absence of illumination, as controlled by notched discs which rotate upon the shafts of the meter register to alternately permit and prevent the transmission of light from an illumination source to the photoresistors, in response to rotation of the register shafts. It is well known to connect the sensor array as a matrix to enable faster, more efficient sampling of the sensors. Examples of arrays having photoresistive sensors are described in U.S. Pat. Nos. 4,037,219, 4,137,451, 3,806,875, 3,662,368, and 3,573,773.
An improved meter dial encoder utilizing a photoresistive sensor matrix array is described in U.S. Pat. No. 4,374,384, which issued to Moates on Feb. 15, 1983. In the encoder disclosed therein, the sensors are arranged in a row and column matrix and include apparatus to maintain a minimal voltage difference between the matrix columns. This prevents undesirable cross-coupling between the columns to eliminate the need for separate isolation elements as was sometimes required in other matrix array encoders. The encoder disclosed in the aforementioned U.S. Pat. No. 4,374,384 is responsive to the current flow through the individual photoresistors, which varies in response to changes in illumination which is incident thereon.
The photoresistors vary their resistance according to the degree of illumination which is incident thereon. However, the material utilized in the photoresistors also varies in its reaction to specified amounts of light. For example, cadmium sulfide, the photoresistive material used in the sensor array of the aforementioned U.S. Patent, has a room temperature dark resistance which can vary from 10 megohms to greater than 1,000 megohms. The resistance of each cadmium sulfide photoresistor when fully illuminated is a much lower value at room temperature, ranging from approximately 10 kilohms to 1 megohm. An additional factor which causes differing resistance of illuminated photoresistors is the differing amount of light which is actually incident upon the photoresistor when the notched code wheel is not blocking the source of illumination. This can result from unequal losses in the various light paths from illumination source to photoresistor and from variations in the strength of the illumination source itself.
A crucial function of the encoder is to definitively determine whether a photoresistor is in an illuminated or non-illuminated state from the resistance of the photoresistor. This is accomplished by establishing a threshold value such that when the resistance of a photoresistor is below the threshold value it is considered to be illuminated and when its resistance is above the threshold value it is considered to be dark.
One method of establishing the threshold is to employ a reference photoresistor which is known to be illuminated. In the encoder disclosed in the aforementioned U.S. patent application, this reference photoresistor was deposited on the same photoresistor array as the photoresistors connected in the matrix, but was not itself matrix connected. Since the reference cell and the matrix cells were formed from a common chemical batch and deposited in a common operation, the photoresistance characteristics of the cells could be assumed to be quite similar. However, some variation in properties does occur even between individual photoresistors on a commonly deposited array, and, as variously mentioned, varying light paths may cause differing amounts of incident radiation between photoresistors, both of which are in an illuminated condition.
In order for the photoresistors to provide a reliable indication of the angular positions of the code wheels, it was therefore necessary to exercise considerable care in the construction of the arrays and in the selection of arrays to actually be used, to thus insure very tight tolerances on the array electrical parameters. Similar care was necessary to insure a uniform attenuation of various light paths. Such measures naturally increase the cost of manufacture.
In some prior art encoder circuits, the selection circuitry would scan a subset of the matrix sensors, either singly or in groups, to determine whether or not the sensors were in the presence or absence of the sensed condition. The non-selected sensors were held in a known high impedance state. This tended to produce susceptibility to noise impulses.
It is therefore an objective of the present invention to provide an encoder for a sensor array which can accurately determine the presence or absence of a sensed quantity using sensors having a larger variation in response to a common sensed condition.
It is a further objective of the present invention to provide an encoder which exhibits higher immunity to noise impulses.